1. Field of the Invention
This invention pertains generally to brake assemblies and air brake systems for heavy duty vehicles. More particularly, this invention pertains to a new air-applied, mechanically-held brake assembly and a new air brake system incorporating the new brake assembly for use with tractors, trailers, and tractor-trailer combinations.
2. Description of the Prior Art
Heavy duty vehicles such as tractors, trailers, and tractor-trailer combinations generally employ braking systems which operate on pressurized air. The vehicle brakes in such systems are controlled by the operation of brake assemblies which operate in response to the supply and exhaust of air under pressure received to and from the rest of the system. The industry is heavily regulated by the national governments of most developed countries and, as a consequence of these regulations, two types of brake assemblies have evolved. These two types are called "spring brakes" and "air-applied, mechanically-held brakes".
Spring brakes are so-called because they are activated for parking and emergency applications by large springs in the brake assembly. In a braking system employing spring brake assemblies, the brakes are applied and released for service applications by the supply and exhaust of air, respectively, to and from a first pressure chamber (usually called the "service chamber") of the brake assembly. The diaphragm of the first pressure chamber operates against a push plate to overcome the force of a return spring and apply the brake when air is supplied to the first pressure chamber. When air is exhausted, the spring returns the push plate to its original position thereby releasing the brake.
The brake of a spring brake assembly is applied and released for parking and emergency applications by the exhaust and supply, respectively, of air under pressure from and to a second pressure chamber of the brake assembly (usually called the "park chamber"). Thus, the exhaust of air releases a large spring to operate against a push plate to apply the brake. The brake is released by the supply of air under pressure to the same diaphragm, which operates against the push plate to overcome the force of the spring to release the brake.
The large springs in a spring brake assembly create several practical problems for the industry. The springs are typically heavy and bulky, and so spring brake assemblies are correspondingly heavy and bulky. These undesirable characteristics detract from the overall performance of the brake assembly and hence the entire brake system. This is especially true for the brake systems requiring relatively large numbers of brake assemblies to comply with governmental regulations for heavy duty vehicles. Spring brakes are also subject to weakened stroke, dragging and broken springs. Furthermore, spring brakes can be dangerous to maintain because of the large spring force, a problem which persists even when the assembly is properly disposed of, and in operation because they can be partially applied without the operator's knowledge. Newly manufactured spring brake assemblies pose additional problems because they are sealed when manufactured, thereby preventing, or at least seriously complicating, maintenance of the brake assembly.
Air-applied, mechanically-held brake assemblies overcome the above problems presented by spring brake assemblies. An air-applied, mechanically-held brake assembly applies and releases the brake in response to the supply and exhaust of air, respectively, to and from a pressure chamber much in the manner of the service chamber of a spring brake assembly. Typically, a diaphragm operates against a push plate upon the supply of air under pressure to overcome the force of the return spring to apply the brake, and, upon the exhaust of the air under pressure, the spring operates to return the push plate to its original position to release the brake. However, the brake is locked in an applied position by some mechanical means in response to the supply and exhaust of air. The mechanical means may vary depending upon the particular design of the brake assembly but all such mechanical means generally have the advantage of overcoming the problems associated with spring brake assemblies.
Such air-applied, mechanically-held brake assemblies are disclosed, along with an accompanying air brake systems, in the U.S. Patent Re. No. 29,913 issued Feb. 20, 1979 to Case et al., in U.S. Pat. No. 4,685,744 issued Aug. 11, 1987 to Ronald W. Luce. In the latter, air is supplied to and exhausted from the first pressure chamber of the brake assembly via a two-way shuttle valve. Once pressure is supplied to the pressure chamber and the diaphragm operates against the push plate to overcome the force of the return spring to apply the brake, air is exhausted from a piston in the brake assembly which has a pawl on the end of it. The piston drops when the air pressure is exhausted therefrom to permit the pawl to be spring pressed into engagement with a rack on the push rod attached to the push plate, thereby locking the brake in an applied position. The brake can be unlocked by supplying air under pressure to the piston to raise the piston and disengage the pawl on the end of the piston from locking engagement with the rack on the push rod. The brakes are thereby "mechanically held" by the rack and pawl engagement of the push rod and piston.
Another air-applied, mechanically-held brake assembly is disclosed in United States Statutory Invention Registration H748 obtained by John M. Graham and published Mar. 6, 1990. Graham employs a collet operating on the outside of the push rod driven by a diaphragm in response to the supply and exhaust of air under pressure to lock the brake in an applied position. In theory, the collet is driven into a tapered hole by the force of a spring when pressure is exhausted from the diaphragm, thereby clamping the push rod and preventing further movement. The brakes are consequently "mechanically held" in an applied position by the collet. However, as will be subsequently discussed, the design disclosed in Graham suffers from several fundamental flaws.
A third air-applied, mechanically-held brake assembly is the DD-3 brake assembly manufactured and sold by the Heavy Vehicle Systems Group of Bendix. The DD3 has a diaphragm which overcomes the force of a return spring on the supply of air under pressure. When air is exhausted from a piston operating the locking mechanism, the biasing force of a spring drives a plurality of rollers against an inclined surface that forces the rollers against the outside of the push rod to bind further movement. The push rod is thereby "mechanically held" by the binding force generated by the interaction of the rollers and the inclined surface.
One problem common to these and other mechanically-held brake assemblies not present in spring brake assemblies is back pressure on the push rod resulting from the cooling of heated brake drums. The drums of a vehicle's brakes become heated and expand during periods of intense or prolonged use. If the brakes are locked in an applied position while the brake drums are heated and are left locked for a period of time, the brake drums will cool and contract, thereby generating a back pressure on the push rod. The back pressure effectively magnifies the holding force locking the brakes, and creates severe problems unless adequately dealt with.
The control valve disclosed and claimed in U.S. Pat. No. 4,907,814 issued to International Transquip Industries, Inc. as the assignee of the inventor Nathan L. Goldfein on Mar. 13, 1990 addressed the back pressure problems facing the Case et al. and Luce brake assemblies. Although the brake assemblies work well in air brake systems with conventional control valves, the Goldfein control valve increases performance by manipulating pressures within the air brake system so that air at the peak pressure found on the diaphragm is used to allow the piston to raise and unlock the brakes. A greater force is therefore available to unlock the brakes than is found in the combined braking force exerted in applying the brake and the back pressure created by contracting brake drums.
The Graham brake assembly disclosed in Statutory Invention Registration H748 was apparently inspired by the need to effectively cope with additional forces generated by back pressure. The specification accompanying the Statutory Invention Registration H748 indicates that Graham intended that the locking means be axially movable in the brake "off" position in response to the presence of back pressure. However, among other things, the collet diaphragm is pierced through in several places in a manner which may prevent maintaining a seal at the necessary operating pressures.
It is therefore an object of this invention to satisfy both the technical and economical considerations in a manner which complies with governmental regulations and operate with currently used and installed air brake systems.
It is also an object of this invention to provide an operable air-applied, mechanically-held brake assembly for use with heavy vehicles which adequately deals with back pressure on the push rod.
It is furthermore an object of this invention to provide such an air-applied, mechanically-held brake assembly which is usable in air brake systems employing conventional valving.